A 72-year-old woman is evaluated in the hospital for acute kidney injury. She was admitted… — USMLE Step 3

A 72-year-old woman is evaluated in the hospital for acute kidney injury. She was admitted 6 days ago for urosepsis and has been treated with intravenous ampicillin, gentamicin, and intravenous fluids. Over the past 24 hours, her urine output has decreased to 400 mL. Her blood pressure is 122/76 mm Hg, and heart rate is 84/min. She appears euvolemic, and her mucous membranes are moist. Laboratory studies show:

Parameter	Result
Serum chemistry
Sodium	$140 \text{ mEq/L}$
Potassium	$4.5 \text{ mEq/L}$
Chloride	$105 \text{ mEq/L}$
Bicarbonate	$22 \text{ mEq/L}$
Blood urea nitrogen	$35 \text{ mg/dL}$
Creatinine	$2.5 \text{ mg/dL}$
Urine studies
Sodium	$56 \text{ mEq/L}$
Creatinine	$50 \text{ mg/dL}$

Based on these laboratory results, what is the patient's fractional excretion of sodium (FENa) in percent?

Answer: 2 %

Answer

The patient's fractional excretion of sodium (FENa) is 2%.

The correct calculated value is 2%. The fractional excretion of sodium (FENa) measures the proportion of sodium filtered by the glomeruli that is ultimately excreted in the urine. It is widely utilized to differentiate between prerenal and intrinsic causes of acute kidney injury (AKI). In prerenal AKI, the kidneys avidly reabsorb sodium to restore intravascular volume, leading to a FENa < 1%. Conversely, in intrinsic renal disease such as acute tubular necrosis (ATN), the damaged tubules lose their ability to concentrate urine and reabsorb sodium effectively, resulting in a FENa typically > 2%. In this vignette, the patient developed AKI after exposure to gentamicin, a classic nephrotoxic agent known for causing ATN. Her calculated FENa of 2% corroborates this intrinsic tubular injury mechanism.

Step-by-Step Solution

Identify the standard formula for calculating the Fractional Excretion of Sodium (FENa).

\text{FENa} = \frac{\text{Urine Na} \times \text{Serum Cr}}{\text{Serum Na} \times \text{Urine Cr}} \times 100

This equation calculates the percentage of filtered sodium that is eventually excreted in the urine, serving as a critical index to differentiate the etiology of acute kidney injury.

Extract the specific necessary laboratory values from the clinical vignette.

\text{Urine Na} = 56 \text{ mEq/L}

\text{Serum Cr} = 2.5 \text{ mg/dL}

\text{Serum Na} = 140 \text{ mEq/L}

, and

\text{Urine Cr} = 50 \text{ mg/dL}

Accurate variable substitution is required to compute the index correctly.

Perform the mathematical calculation.

\text{FENa} = \frac{56 \times 2.5}{140 \times 50} \times 100 = \frac{140}{7000} \times 100 = 0.02 \times 100 = 2\%

Solving the equation yields the final physiological percentage used for diagnostic reasoning.

Key Concept

Fractional Excretion of Sodium (FENa) calculation and interpretation for distinguishing intrinsic acute kidney injury from prerenal disease.

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